Most modern, mid-range to high-end disk storage subsystems are arranged as redundant arrays of independent disks (RAID). A number of RAID levels are known. RAID-1 includes sets of N data disk drives and N mirror disk drives for storing copies of the data disk drives. RAID-3 includes sets of N data disk drives and one parity disk. RAID-4 also includes sets of N+1 disk drives, however, data transfers are performed in multi-block operations. RAID-5 distributes parity data across all disk drives in each set of N+1 disk drives. At any level, it is desired to have RAID systems where an input/output (I/O) operation can be performed with minimal operating system intervention.
When a drive fails, the redundant data are used to reconstruct all of the failed data on the array. While the RAID subsystem is reconstructing the data of the failed drive for the user, the RAID array is vulnerable to a second disk failure. When the array is in this state of reconstruction, it is known as a degraded array, as opposed to being fully protected.
Because degraded arrays can cause failure of services to a user with only one drive failure, it is imperative to minimize the time the array is in the degraded state. To alleviate this problem, RAID subsystems use the concept of a hot spare space. Spare space can either be allocated on a dedicated spare disk, or allocated in a distributed manner over all of the active disk drives of the array. When the array is in the degraded state, the RAID subsystem can immediately begin to repair the array by generating a new set of user and redundant data in the spare space. When the rebuild is complete, the RAID array is again in a fully protected state.
FIG. 1 shows a RAID array 110 with spare space 100. The RAID array 110 can failover 120 to the spare space 100 should one of the disk drives 111–116 fail. The spare space 100 can be located on the disk drives 111–116 or on a dedicated disk drive.
Unfortunately, the spare space has drawbacks because it leaves a large amount of disk space unused. Some RAID subsystems have tried to use this empty space in a very narrow fashion. See for example, U.S. Pat. No. 5,666,512 issued to Nelson on Sep. 9, 1997 “Disk Array Having Hot Spare Resources To Store User Data.”
In addition to the spare space being part of most modern RAID subsystems, special features such as RAID level migration, i.e., changing RAID levels, and RAID array expansion, i.e., making RAID arrays larger, are becoming basic requirements in any RAID implementation. The functionality of changing RAID levels and array size expansion should be dynamic, allowing users to access the data while these changes take place.
RAID array expansion, for RAID levels which stripe across disk drives in particular, can be a difficult task. In known RAID subsystems, the expansion takes place by adding a new disk, blocking access to all of the new space on the new disk and then distributing the data in the RAID array to the expanded array of disk drives, while the additional new disk cannot be used. Only when the distribution is complete, can access to that new space be permitted so that new data can be stored again.
The reason why the new disk cannot be used for new data are that the new data would need to be stored someplace. Attempting to store the data on the new disk, and then distributing the data cannot be done because the distributed data would overlap both old and new data mapping.
When an array needs to be expanded, it presents a difficult situation, especially when files storing old data cannot be deleted, and there is insufficient storage for new data. Even with modern RAID arrays, which do not block data access while expanding, the process of redistributing the old data can take many hours, or perhaps even days, under extreme I/O load conditions. The inability to add new data during this time would be a severe constraint in any file or database system where the “turn-over” of data are short-term.
Therefore, there is a need for a system and method for expanding a RAID subsystem in such a way that access to both old and new data are not interrupted.